1. Field of the Invention
The present invention relates to multi-color electrophotographic machines, and, more particularly, to setting laser power and developer bias in multi-color electrophotographic machines.
2. Description of the Related Art
Toner patch sensors are used in color printers and copiers to monitor and control the amount of toner laid down by the electrophotographic process. Toner patch sensors reflect light off of a toner patch to determine how much toner was laid down during the electrophotographic process. The sensor""s voltage signal from reading a toner patch is compared to the sensor signal from reading a bare surface to produce either a voltage difference or a ratio between the two signals.
Toner patch sensors are used in printers and copiers to monitor the toner density of unfused images and provide a means of controlling the print darkness. This information is then used to adjust laser power, developer bias, and other process conditions that affect image density. In color printers and copiers, the toner patch sensors are used to maintain the color balance and in some cases to modify the gamma correction or halftone linearization as the electrophotographic process changes with the environment and aging effects. Conventional reflection based toner sensors use a single light source to illuminate a test patch of toner and one or more photosensitive devices to detect the reflected light.
It is known to use the test images to both control solid area density and to apply a gradation correction (linearization) to the halftone printing curve. The automatic color adjustment process can be annoying to the printer user since the printer is unavailable for printing customer jobs for several minutes during this process. The test patches used in this process also consume toner, which reduces the cartridge yields and increases the need for waste toner storage.
It is known for a printer to have at least two modes of print resolution, such as 600 dots per inch (dpi) and 1200 dpi. The halftone screens used in each mode may use different dot sizes and screen angles. Because of this and the change in process speed, the gradation correction required for the 600 dpi mode halftones is different than the gradation correction needed for 1200 dpi halftones. FIG. 1 is a plot of L* (lightness) values for a range of cyan halftone values (0-255) printed in 600 dpi mode and in 1200 dpi mode. A plot such as FIG. 1 is known as the xe2x80x9chalftone printing response curvexe2x80x9d, xe2x80x9chalftone response curvexe2x80x9d, or xe2x80x9chalftone printing curvexe2x80x9d. The 1200 dpi halftones print noticeably lighter than the 600 dpi halftones for halftone levels of 20-50. Experiments have shown that the 1200 dpi halftone densities cannot reliably be predicted from the 600 dpi halftones because the correlation between the two halftone series varies with laser power and developer bias.
The correlation between the two modes will also vary with the frequency response of the particular laser printheads used in a given printer. To achieve the highest possible print quality, it then becomes necessary to perform additional reflective measurements on patches in 1200 dpi mode. This additional information can then be used to perform an accurate gradation correction on the 1200 dpi halftones. Because the 1200 dpi halftones are printed at a reduced speed, the total time required and waste toner generated by this double color adjustment procedure is even more annoying to the average user.
What is needed in the art is a method of providing accurate color adjustments that minimizes the time and toner used by the color adjustment procedure when there are multiple print resolution modes.
The present invention provides a method of using a toner patch sensor to control the solid area density and provide information for linearizing the halftone response at 600 dpi. Additional patches are printed at 1200 dpi only as needed to allow the printer to correctly linearize the 1200 dpi halftone response. The 1200 dpi set points for laser power and developer bias are extrapolated from the 600 dpi set points.
The invention comprises, in one form thereof, an electrophotographic machine having at least two printing modes, with each printing mode having a respective printing density. A method of calibrating the electrophotographic machine includes depositing at least one toner patch on an image-bearing surface. The depositing is performed in a first of the printing modes. Light is emitted onto the at least one toner patch. An amount of light that is reflected off of the at least one toner patch is measured. At least one first electrophotographic condition for the first printing mode is adjusted dependent upon the measuring step. At least one second electrophotographic condition is adjusted for a second of the printing modes. The adjusting of the at least one second electrophotographic condition is dependent upon the measuring step.
The invention comprises, in another form thereof, a method of calibrating an electrophotographic machine having at least two printing modes. Each printing mode has a respective printing density. The method includes depositing at least one solid area toner patch on an image-bearing surface. The depositing is performed in a first of the printing modes. Light is emitted onto the at least one solid area toner patch. An amount of light that is reflected off of the at least one solid area toner patch is measured. At least one first electrophotographic condition for printing in the first printing mode at full density is adjusted dependent upon the measuring step. At least one first halftone patch is deposited on the image-bearing surface. The depositing is performed in the first printing mode and is dependent upon the at least one first electrophotographic condition. Light is emitted onto the at least one first halftone patch. An amount of light that is reflected off of the at least one first halftone patch is measured. A correction curve of gradation for printing in the first printing mode at less than full density is formed dependent upon the measured amount of light that is reflected off of the at least one first halftone patch. At least one second electrophotographic condition for printing in a second of the printing modes at full density is adjusted dependent upon the measured amount of light that is reflected off of the at least one solid area toner patch. At least one second halftone patch is deposited on the image-bearing surface. The depositing is performed in the second printing mode and is dependent upon the at least one second electrophotographic condition. Light is emitted onto the at least one second halftone patch. An amount of light that is reflected off of the at least one second halftone patch is measured. A correction curve of gradation for printing in the second printing mode at less than full density is formed dependent upon the measured amount of light that is reflected off of the at least one second halftone patch.
An advantage of the present invention is that color adjustments for two different printing modes can be made by printing toner test patches in only one of the two modes.
Another advantage is that a minimal amount of time is required to perform color adjustment for two different printing modes.
Yet another advantage is that a minimal amount of toner is required to perform color adjustment for two different printing modes.